java - how does the method infer the type of <T>

Question

The below method works flawlessly

public <T> void fromJsonArray(String jsonString,Type tToken) {
  Gson g = new Gson();
  T list = g.fromJson(jsonString,tToken);
  System.out.println(list);
}

But i have not specified what < T > is in this method. how does the compiler assign the value returned by the fromJson method to the variable list whose type i have not specified?

i just tested the validity of the answers stating about <T> being inferred from the return type of the method. It doesn't seem to work out. Please check out the following code. it doesn't even compile

import java.util.*;

class Sample {

  public List<String> getT(String s) {
    List<String>  list = new ArrayList<String>();
    list.add(s);
    return list;
  }

  public <T> void test(){
    T list = getT("test");
    System.out.println(l);
  }

  public static void main(String[] a) {
    new Sample().test();
  }
}

modified the source again and tested it, and it resulted in a compile time error

public <T> List<T> getT(T s) {
  List<T>  list = new ArrayList<T>();
  list.add(s);
  return list;
}

public <T> void test(){
  T list = getT("test"); //incompatible types compilation error here
  System.out.println(list);
}

Sample1.java:13: error: incompatible types T list = getT("test"); ^ required: T found: List where T is a type-variable: T extends Object declared in method test()

Answer 1

how does the method infer the type of <T>

It doesn't. Generic methods don't infer their generic types - that's why T is called a type parameter. The caller of the method provides a type argument for T. When it does, it may be inferred by the compiler based on the context of the method call's arguments and target type.

For example:

Set<String> c = Collections.emptySet();

emptySet declares a type parameter T, takes no arguments, and returns a Set<T>. Here, the compiler infers T to be String based on the target type, Set<String>.

Another example:

Collections.singleton("asdf");

singleton declares a type parameter T, takes a T, and returns a Set<T>. Here, there is no target type, but the compiler infers T to be String based on the argument "asdf".

But generic type inference is just a convenience. Without it, we could still use type witnesses to explicitly provide type arguments:

Set<String> c = Collections.<String>emptySet();
Collections.<String>singleton("asdf");

This brings us to your method signature:

public <T> void fromJsonArray(String jsonString, Type tToken)

fromJsonArray declares a type parameter T, but doesn't return anything related to the type T or take arguments related to T. At a call to fromJsonArray, the compiler has no information from which to infer T. Its type argument will default to its upper bound Object unless a type witness is used:

myObj.<String>fromJsonArray(jsonString, tToken);

But this doesn't matter because <String> has no affect on the behavior of the method call or its compilation. T is meaningless* and can be removed from the declaration of fromJsonArray.

how does the compiler assign the value returned by the fromJson method to the variable list whose type i have not specified?

Here is the source of Gson.fromJson(String, Type):

@SuppressWarnings("unchecked")
public <T> T fromJson(String json, Type typeOfT) throws JsonParseException {
    StringReader reader = new StringReader(json);
    T target = (T) fromJson(reader, typeOfT);
    return target;
}

You can see it declares an arbitrary type parameter T and casts the deserialized object to T. This is known as an unchecked cast, because it won't fail fast if it's wrong. That's because T has been erased at runtime. You can see that the code is suppressing a warning about doing this, because it's generally a bad idea. By not restricting what T is based on the method arguments, the Gson code has effectively ceded control over it to the caller. If you wrote:

List<String> list = g.fromJson(jsonString, tToken);

but tToken represented HashSet<String>, you would get a ClassCastException on that line at runtime. Worse, if tToken represented ArrayList<Integer>, it would not even fail on that line, because the JVM would only see a List and allow the assignment to happen. A ClassCastException would be thrown sometime later, once your code tried to treat the list's Integer elements like Strings (and the exception would be confusing to debug).

So to answer your question about the assignment, the compiler lets you assign the result of fromJson to anything you want. It's up to you for it to be correct.

You may ask, Why would Gson do an unchecked cast and allow unsafe code? The answer is that it's a convenience, stemming from language limitations. Their other signature is safer:

public <T> T fromJson(String json, Class<T> classOfT)

But there's no way to represent generic types with Class - no List<String>.class for example. Only a Type can do this, and it's not itself generic. fromJson could have required a TypeToken<T>, but there are other ways to obtain a Type, so that would be restrictive.

Returning Object and forcing the caller to do the unchecked cast would have been more transparent but the Gson developers probably wanted to avoid this "ugliness".